(*FAIL) (*F)
This pattern matches nothing and always fails. It can be used to force+ the engine to backtrack. It is equivalent to (?!), but easier to rea+d. In fact, (?!) gets optimised into (*FAIL) internally.

The non-greedy quantifier in (\w{2,}?) is redundant, since the forced backtracking ensures that all substrings will be found whether the search is greedy or not.

If, as per your sort, you are looking for only the shortest substring with the maximum frequency, then you need only search for substrings exactly 2 characters long (the minimum length): (\w{2}). (Proof: Say XYZ occurs N times. XY and YZ occur once in each occurrence of XYZ, so each is guaranteed to occur at least N times in the string. Similar reasoning applies to longer substrings.)

++trizen for demonstrating this elegant technique to force regex backtracking!